Bone conduction hearing devices are used by patients who can not use conventional air conduction hearing aids e.g., due to chronic middle ear disease or a congenital/acquired deformity.
A traditional low cost bone conduction hearing device consists of a bone conduction transducer enclosed in a plastic housing which is pressed with a constant pressure of 3-5 Newton against the skin over the bone behind the ear. Microphone, amplifier, and power source are placed in their own housing at a suitable site and at a secure distance from the transducer to avoid feedback problems. The most essential drawbacks of this type of bone conduction hearing devices are that it is uncomfortable to wear due to the constant pressure and that the soft skin over the bone deteriorate the transmission of vibrations to the bone.
Since the beginning of the 1980's there is a second type bone conduction device—the bone anchored hearing aid (BAHA)—where the bone conduction transducer is connected directly to the bone via a skin penetrating and bone anchored implant of titanium, cf e.g., SE8107161, SE9404188 or Tjellström et al. 2001. In this way a bone conduction hearing device is obtained which provides higher amplification, improved wearing comfort, and where all parts can be enclosed in the same housing.
In the future there may be a third generation of bone conduction hearing devices where the transducer is supposed to be implanted completely and thereby skin and soft tissue can remain intact. Signal and necessary energy can in this case be transferred through intact skin by means of inductive coupling, as described by Håkansson et al. 2008. At more severe hearing damages where the energy demand is large the energy can be transferred by means of skin penetrating (percutaneous) electric connection device, cf e.g., SE9704752. The advantages implanting the whole transducer into the temporal bone compared with a transducer being externally situated are, besides the pure medical ones, that an increased sensitivity is obtained, the size of the externally placed unit becomes smaller and stability margins are improved.
It is of course of utmost importance that all bone conduction transducers in general and implantable ones in particular are efficient and keep current consumption low and that the sensitivity i.e. output force over the whole frequency range is high enough.
To achieve sufficiently high low frequency sensitivity conventional transducers are designed to have a first resonance created from the interaction between the counterweight mass and the suspension compliance (elasticity). Both the mass and the compliance are also needed from inherent reasons i.e. the suspension compliance is needed to prevent air gaps from collapsing and the counter weight mass is needed to induce the forces created in the airgap to the load. This low frequency resonance is typically placed somewhere between 200-1000 Hz and gives the transducer a low frequency sensitivity boost. However, it is well known that bone conduction devices suffer from a limited maximum output at high frequencies, especially if compared with air conduction devices. To improve the sensitivity of bone conduction transducers in the high frequency area is the major objective behind the present invention.
The present innovation is also applicable to other applications than bone conduction hearing aids such as transducers for bone conduction communication systems, audiometric and vibration testing devices.